Dynamic fluorescence measurements of radiotionless energy transfer between tryptophan donor groups located within the regulatory peptides Phk13 and Phk5 of glycogen phosphorylase kinase and nitrotyrosine acceptor groups within calmodulin or Phk13 have provided a set of separations and separation distributions which place definite restrictions upon the geometry of the peptide-calmodulin complexes. Phk13 appears to be bent into a hairpin-shaped beta-structure. The distribution of separations are suggestive of substantial internal mobility of the complex species.